1. Field of the Invention
The present invention relates to a method for charging a rechargeable battery and a protection circuit for the rechargeable battery and, more particularly, to a pre-charge method capable of limiting the amount of electric current input through a charger at the initial stage of charging and a protection circuit for the rechargeable battery.
2. Discussion of Related Art
The process of charging and discharging a rechargeable battery, on the basis of the state of charge of the battery, is performed during the usage of the rechargeable battery, such as a lithium ion battery. Because energy is stored in batteries, if problems such as an internal short circuit occurs, ignition or explosion may occur as the stored energy is released in a short period of time. Particularly, in case of a lithium rechargeable battery, improving safety is increasingly needed because the energy density of the lithium rechargeable battery is high and non-aqueous electrolyte used in the lithium rechargeable battery is inflammable.
As a device for improving safety, a positive temperature coefficient thermister interrupting the current by monitoring the temperature change, or a circuit breaker using a bimetal, or a safety vent preventing the explosion by monitoring the internal pressure change, etc., may be used, and a protection circuit preventing overcharge is usually installed in the charger or the battery itself.
A protection circuit for the contemporary rechargeable battery is typically constructed with a bare cell, a charging field-effect transistor (hereinafter, referred to as “FET”) which is connected with in series the bare cell and which is turned off in case of overcharging and overdischarging, a discharging field-effect transistor, a fuse that breaks the circuit when the internal temperature exceeds a certain value, a sensor register sensing the current of the bare cell, a main control circuit generating various control signals by integrating electrical signals from the bare cell and the sensor register, a first protection circuit either turning on or turning off the charging FET or the discharging FET, and a second protection circuit connected to the fuse via the main control circuit and either turning on or turning off a short circuit preventing FET. The bare cell is a battery in a state where a cap assembly is coupled with a can into which an electrode assembly is inserted. In other words, the bare cell shows the state of the rechargeable battery before the rechargeable battery is formed as a cylindrical type battery pack, or an angular type battery pack, or a pouch type battery pack. All constituent elements are build into a single battery pack. An external charger may charge the rechargeable battery, and an external driving load may be connected to the battery when the rechargeable battery is discharged.
In the protection circuit for the contemporary rechargeable battery, when the bare cell is overcharged, the sensor register senses the state of overcharging and outputs certain electrical signals to the main control circuit. The main control circuit outputs control signals to the first protection circuit. And then, the first protection circuit turns off the charging FET by applying low signals to the gate electrode of the charging FET, and charging of the battery is therefore stopped.
In case that the bare cell is over-discharged, discharging of the battery is stopped by turning off the discharging FET through a similar control path as described above.
In case that the internal temperature of the rechargeable battery rises due to the occurrence of an abnormal state such as internal short circuit, an ignition and explosion of the rechargeable battery are prevented by activating the second protection circuit connected to a temperature sensitive fuse. The temperature sensitive fuse may melt when the temperature rises to a certain level, thus cutting off the current flowing into the circuit.
Meanwhile, the charger usually provides constant charging current at the initial stage of charging and constant voltage at the latter period of charging. For the lithium rechargeable battery, if constant charging current at the initial stage of charging is provided, the internal structure of a negative electrode active material inside the battery might be destroyed and lifetime of the battery might be reduced because the current value will rise to exceed an appropriate level. To resolve these problems, at the initial stage of charging, the charging FET is turned off, a pre-charging FET and a posister, that is, a positive temperature coefficient thermister, are installed in a bypass circuit to induce the charging current. And the charging current, which flows through the pre-charging FET, is limited within an range by the resistance of the posister, so that the charging current does not deleteriously effect the rechargeable battery electrode structure.
After a certain time passes, or when the voltage of the rechargeable battery bare cell reaches a certain level, and the pre-charging FET is turned off and the charging FET is turned on by adjusting the gate voltage to induce the charging current.
In the contemporary protection circuit, however, costs of the posister and the pre-charging FET of the bypass circuit are high, thereby increasing the cost of a protection circuit board and the manufacturing cost of the rechargeable battery.